The present invention relates to a hydraulic transmission device capable of acting as either a hydraulic pump which converts mechanical energy to hydraulic energy or as a hydraulic motor which receives hydraulic energy and converts it to mechanical energy, and more particularly to a swash plate type axial piston pump-motor having a swash plate provided at a tilt angle.
A swash plate type axial piston pump-motor has conventionally been known as a hydraulic transmission device which can be used in converting received mechanical energy to hydraulic energy or, conversely, in obtaining mechanical energy from hydraulic energy. There are two types of swash plate, axial piston pump-motors; one is a constant displacement type axial piston pump-motor which has a swash plate provided at a constant tilt angle, and the other is a variable displacement type axial piston pump-motor which enables the tilt angle of the swash plate to be varied. There is no particular difference in energy conversion principle between these types.
FIG. 1 shows an example of a conventional variable displacement type of axial piston pump-motor, that disclosed in Japanese Patent Publication No. 39569/1970. In this example, one end of a housing 14 (i.e., the left end as viewed in the drawing) is closed and has in a substantially central location a through hole for inserting a drive shaft, while the other end of the housing 14 (i.e., the right end as viewed in the drawing) is open. The drive shaft 11 is connected to a power source such as a motor (not shown), is inserted into the through hole, and is journaled at one end in the housing 14 by means of a bearing 16. A cylinder block 13 is splined on the drive shaft 11 and is provided with a plurality of cylinders 18A which are angularly spaced around the longitudinal axis of rotation of the drive shaft and are disposed parallel to the axis. A plurality of pistons 18 are each received in a corresponding cylinder 18A so as to reciprocate therein, and each has an end (i.e., the left end as viewed in the drawing) formed as a spherical head which rotatably engages with a corresponding shoe 19 that is kept in substantially continuous contact with the planar surface of a tiltable swash or cam plate 20 as the cylinder block 13 rotates.
One end of the cylinder block 13 (i.e., the left end as viewed in the drawing) is in engagement with a retainer 12 which has a spherical outer surface and is provided on the shaft 11, so as to support the swash plate 20 through the shoes 19 and a return plate 22. The tilt angle of the swash plate 20 can be varied by means of an actuator incorporated in the housing.
The end portion of the housing 14 at its open end is closed by a rear cover 15 over a port plate (valve plate) 23, and the other end of the cylinder block 13 can slide on the port plate 23. Substantially central locations of the port plate 23 and the rear cover 15 have coaxial holes into which the other end 11A of the drive shaft 11 is inserted so as to be supported by the rear cover 15. The cylinder block 13 which is splined on the drive shaft 11 is supported in this way in the housing 14. The port plate 23 and the rear cover 15 are provided with a plurality of intake and discharge ports 27 and 26 communicating with the corresponding cylinders 18A, so that the device can act as an axial piston pump by sucking in and discharging operating fluid from and to the exterior when the drive shaft 11 is rotated by the operation of a drive source, causing the rotation of the cylinder block 13 and reciprocal movement of the pistons 18; or, conversely, as an axial piston motor when the suction and discharge of operating fluid causes the rotation of the drive shaft.
However, with the conventional swash plate type axial piston pump-motor having the above-described arrangement, the cylinder block 13, the drive shaft 11, and the retainer 12 are respectively composed of separate component parts, and the retainer 12 is urged by springs 30 provided within the cylinder block 13 toward the annular, shoe-contacting return plate 22. Consequently, the overall structure of the device is complicated and its production costs are therefore high. In addition, since the cylinder block 13 and the drive shaft 11 are composed of individual component parts, small gaps may occur between these members during assembly, causing relative movement thereof and, hence, noise when there is a variation in the load.
In view of these circumstances, various proposals have been made, including a proposal for an arrangement in which the cylinder block, the drive shaft, and retainer are designed to have an integral structure, in an attempt to solve the above-described problems. With this proposal, however, the end surface of the cylinder block 13 (the right end surface as viewed in FIG. 1) that is away from the drive shaft end journaled by the bearing and is in sliding contact with the port plate 23 is supported in the housing 14 by making the rear end portion 11A of the drive shaft 11 project through that end surface of the cylinder block 13 into the rear cover 15. Thus, this support of the right end surface of the cylinder block 13 necessitates the rearwardly projecting end portion 11A of the drive shaft 11. However, the projecting shaft portion represents an obstacle to the machining of the rear end surface of the cylinder block 13 and makes it difficult to work the rear end surface with a very high degree of precision. Consequently, it is nearly impossible to limit the amount of leakage between the end surface of the cylinder block 13 and the port plate 23 to a small amount. Thus, the prior art has not been able to achieve a highly efficient pump-motor.